Beer spoilage organisms: Occurrence and detection with particular reference to a new genus Pectinatus: Dissertation

Lactic acid bacteria of the genera Lactobacillus and Pediococcus, acetic acid bacteria, enterobacteria and wild yeasts of the genus Saccharomyces are regarded as the most harmful beer spoilage organisms due to their ability to cause off-flavours and/or turbidity in beer.The investigation concerning the microbiological status of Finnish breweries revealed that the most frequent contaminant during fermentation was Pediococcus damnosus.Enterobacter agqlomerans was detected in the few brews in which heavy contaminations of Gram-negative species were found.The most frequent source of contamination was the pitching yeast, especially old yeast which had been used in several fermentations.Growth of bacteria occured mainly in yeast storage vessels.Worts were generally free of bacteria and only very low counts of microorganisms were detected in packaged beer.Reasonably low levels of wild yeasts were found in all stages of the brewing process.In general the microbiological status of the breweries investigated was highly satisfactory.In addition to conventional plating methods with selective media, rapid methods based on the use of fluorescing dyes were investigated.The immunofluorescence technique was applied for the detection of wild yeasts in pitching yeast and the microcolony method with optical brighteners was developed for the enumeration of yeasts and bacteria in packaged beer.A new Gram-negative, strictly anaerobic beer spoilage bacterium Pectinatus cerevisiophilus was isolated from turbid beer.The morphological, physiological, biochemical and electron microscopic characterization of this organism revealed only small differences between the Pectinatus isolates studied.The location of flagella on one side of the cell is a distinctive feature of P. cerevisiophilus.Propionic acid, which is the main metabolic product, was produced via the succinate pathway also used by propionibacteria.The formation of other orsanic acids and sulphur compounds resulted in the spoilage of beer f lavour.The cell envelope structure in thin section revealed a thick peptidoglycan layer below the outer membrane and invaginations and mesomes from the cytoplasmic membrane.These are all features normally regarded as typical Gram-positive bacteria. Freezefractured micrographs on the other hand revealed the typical outer membrane of Gram-negative bacteria, with small intramembranous particles (8 - l0 nm).The large intramembranous particles (20 nm) found in the concave faces of the outer and cytoplasmic membranes have not been observed previously in any organisms.On the basis of the immunological characterization using gel-diffusion precipitin tests and immunoelectrophoresis the eleven Pectinatus isolates studied could be assigned to three distinct groups.The chemical composition of the lipopolysaccharides of the Pectinatus isolates revealed an identical fatty acid pattern, indicating that they all belong to a taxonomically homogeneous group.Moreover, the flatty acid composition of Pectinatus isolates distinguishes them from most other groups of Gram-negative bacteria.This, along with the location of flagella, the pathway of propionic acid formation and immunological properties, justifies the separate taxonomic status of this genus.